Sony LCD TV KLV-32BX205 does not boot maintenance example - appliance repair - Huaqiang electronic network

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Repairing a Sony KLV-32BX205 LCD TV, the fault phenomenon: the indicator light is not on, no power on. Initial dismantling of the disassemble, no obvious faults were found. The power-on measurement found that the main circuit of the power board had been activated (STB high-level measured voltage 1.3V) working normally. Further detection found that the power supply voltage through the main board to the screen was not output. The backlight switch control terminal is low level, and there is a set of chip power supply voltage 2.5V (controlled) no output. Since there is no remote control, it is impossible to judge whether the CPU is normal, but after disconnecting the STB control line, it is determined that the power board is normally controlled, and the high level of the power supply is 1.3V from the main board. Since there is no drawing, the cable is filtered back from the screen power supply control circuit. The result shows that the screen power supply and 2.5V pass through the same transistor Q27 and are controlled by the 93 pin of U13, and the 93 pin of U13 is also divided to control the STB of the switch machine. (Draw some sketches as follows), so far from the discovery of the fault should not be far away, when the power is applied to Q27 to detect the voltage, suddenly the backlight is on, hehe! It’s just a good thing to measure such a TV set. It’s really unreliable. After observing for a while, everything is still normal, only the STB becomes 3.3V. When the power is turned off again after power-off, the old problem is restored. It seems that the Q27 is faulty, but there is no problem in carefully detecting Q27. The STB is normal at 3.3V, and there is a problem with 1.3V. Which component is leaking causes the STB voltage to decrease. I detected it on the relevant line, and found no leakage fault, unless there is a problem with the CPU, but I think about it, why is it normal to measure the Q27 TV? I found out that the red test pen was connected to Q27. On the B pole, the TV can work normally. In order to find the culprit exactly, there is no substitute for Q27. After trying different measurements, it is found that the voltage of the transistor Q27BE is about 0.6V, and the Q27 does not have saturation conduction. When the resistor R322 is shorted, Q27 will get more current, and the saturation will be turned on. Finally, the screen power supply and 2.5V normal output, the whole machine will enter the normal working state. After a long time of repeated test, everything is normal.